If your mom ate something different while pregnant with you, such that you
developed with different atoms, does that mean someone else would have been
born in
your place and you wouldn't be conscious? Or if one unexpressed gene was
different, would it be someone other than you looking through those eyes?
What if
one gene were different, but it was of little consequence, or what if
multiple
genes were different, etc. How much of the circumstances would have to
change for
you to never have been born? If you admit that different matter or
different genes
would not make it such that you were never born, then are you not all your
siblings
as well?

That doesn't follow. The most common theory of why you are you is that the
structure of your brain and body encode computations that are peculiar to
you.

If we work from the theory that "you are a computation", there is still the question of
why you are experiencing life as this particular computation vs. that other computation.

But if "you" are a particular computation, the question has a tautological answer. It
would be a contradiction for you to be some other computation.

This is one of the main goals of a theory of personal identity, to rightly delineate
persons and define the scope of experiences that belong to them. Theories of mind and
theories of theories personal identity are related to each other but they are separate
fields.

You are determined by the structure that effects these computations.
This is

independent of the particular atoms and molecules and even a lot of the structure.
As Bruno puts it, it depends on the level of substitution. Just because there is a

level, e.g. atoms, that makes no difference, it doesn't follow that there
is not a
difference at another level.

That was not what I was questioning. My question is more like: if a different sperm
(besides the one that led to you) had made it, what would you expect to be experiencing
right now? Would you expect to be experiencing nothing at all?

The latter, in a metaphorical way, since I wouldn't be expecting or experiencing anything
because this particular "I" wouldn't exist. It's like asking, "If you died in your sleep
would you wake up dead?"